Lake Superior, the largest of the Great Lakes of North America, is the world's largest freshwater lake by surface area, the third largest freshwater lake by volume. The lake is shared by the Canadian province of Ontario to the north, the U. S. state of Minnesota to the west, Wisconsin and the Upper Peninsula of Michigan to the south. The farthest north and west of the Great Lakes chain, Superior has the highest elevation of all five great lakes and drains into the St. Mary's River; the Ojibwe name for the lake is gichi-gami, meaning "great sea." Henry Wadsworth Longfellow wrote the name as "Gitche Gumee" in The Song of Hiawatha, as did Gordon Lightfoot in his song, "The Wreck of the Edmund Fitzgerald". According to other sources, the actual Ojibwe name is Anishinaabe Gichigami; the 1878 dictionary by Father Frederic Baraga, the first one written for the Ojibway language, gives the Ojibwe name as Otchipwe-kitchi-gami. The first French explorers approaching the great inland sea by way of the Ottawa River and Lake Huron during the 17th century referred to their discovery as le lac supérieur.
Properly translated, the expression means "Upper Lake,". The lake was called Lac Tracy by 17th century Jesuit missionaries; the British, upon taking control of the region from the French in the 1760s following the French and Indian War, anglicized the lake's name to Superior, "on account of its being superior in magnitude to any of the lakes on that vast continent." Lake Superior empties into Lake Huron via the Soo Locks. Lake Superior is the largest freshwater lake in the world in area, the third largest in volume, behind Lake Baikal in Siberia and Lake Tanganyika in East Africa; the Caspian Sea, while larger than Lake Superior in both surface volume, is brackish. Lake Superior has a surface area of 31,700 square miles, the size of South Carolina or Austria, it has maximum breadth of 160 statute miles. Its average depth is 80.5 fathoms with a maximum depth of 222.17 fathoms. Lake Superior contains 2,900 cubic miles of water. There is enough water in Lake Superior to cover the entire land mass of North and South America to a depth of 30 centimetres.
The shoreline of the lake stretches 2,726 miles. American limnologist J. Val Klump was the first person to reach the lowest depth of Lake Superior on July 30, 1985, as part of a scientific expedition, which at 122 fathoms 1 foot below sea level is the second-lowest spot in the continental interior of the United States and the third-lowest spot in the interior of the North American continent after Iliamna Lake in Alaska and Great Slave Lake in the Northwest Territories of Canada at. While the temperature of the surface of Lake Superior varies seasonally, the temperature below 110 fathoms is an constant 39 °F; this variation in temperature makes the lake seasonally stratigraphic. Twice per year, the water column reaches a uniform temperature of 39 °F from top to bottom, the lake waters mix; this feature makes the lake dimictic. Because of its volume, Lake Superior has a retention time of 191 years. Annual storms on Lake Superior feature wave heights of over 20 feet. Waves well over 30 feet have been recorded.
The lake is fed by over 200 rivers. The largest include the Nipigon River, the St. Louis River, the Pigeon River, the Pic River, the White River, the Michipicoten River, the Bois Brule River and the Kaministiquia River. Lake Superior drains into Lake Huron via the St. Marys River. There are rapids at the river's upper end where the river bed has a steep gradient; the Soo Locks were built to enable ships to bypass the rapids and to overcome the 25-foot height difference between Lakes Superior and Huron. The lake's average surface elevation is 600 feet above sea level; until 1887, the natural hydraulic conveyance through the St. Marys River rapids determined the outflow from Lake Superior. By 1921, development in support of transportation and hydroelectric power resulted in gates, power canals and other control structures spanning St. Marys rapids; the regulating structure is known as the Compensating Works and is operated according to a regulation plan known as Plan 1977-A. Water levels, including diversions of water from the Hudson Bay watershed, are regulated by the International Lake Superior Board of Control, established in 1914 by the International Joint Commission.
Lake Superior's water level was at a new record low in September 2007 less than the previous record low in 1926. However, the water levels returned within a few days. Historic high water The lake's water level fluctuates from month to month, with the highest lake levels in October and November; the normal high-water mark is 1.17 feet above datum (601.1 ft
The Copper Country is an area in the Upper Peninsula of Michigan in the United States, including all of Keweenaw County and most of Houghton and Ontonagon counties as well as part of Marquette County. The area is so named as copper mining was prevalent there from 1845 until the late 1960s, with one mine continuing through 1995. In its heyday, the area was the world's greatest producer of copper; the Copper Country is unusual among copper-mining districts in that the copper was predominantly in the form of copper metal rather than the copper oxides or copper sulfides that form the copper ore at every other copper-mining district. Native Americans mined copper from small pits as early as 3000 B. C; the Michigan State Geologist Douglass Houghton reported on the copper deposits in 1841. The first successful copper mine, the Cliff mine, began operations in 1845, spurred by venture capital from Boston and other East Coast investors, many other mines followed. Mining of the most productive deposit, the Calumet conglomerate, began in 1865.
Mining took place along a belt. While mining continues on a small scale and logging are now the area's major industries. Popular tourist destinations include the cities of Copper Harbor and Houghton, the Porcupine Mountains with Lake of the Clouds. Snowmobiling is popular in the winter, snowmobile trails are found in most areas. Large numbers of Finns, Danes and Norwegians emigrated to the Upper Peninsula the Keweenaw Peninsula, to work in the mines, and they stayed on and prospered after the copper mines closed. In addition to the aforementioned Nordic peoples, ethnic groups that inhabited the area included: Chinese; the Copper Country is rural, much of it has been designated as state parks or similar designations. These include McLain State Park, Porcupine Mountains State Park, the Copper Country State Forest; the Keweenaw National Historical Park includes several important sites relating to the area's copper-mining history. Institutions of higher education include Finlandia University in Hancock, founded in 1896 as Suomi College, Michigan Technological University in Houghton established in 1885 as the Michigan School of Mines.
Finlandia University is affiliated with the Evangelical Lutheran Church in America, reflecting the spiritual heritage of the region's many Finnish immigrants. Michigan Tech was founded in response to the needs of the copper mines; the Copper Country averages more snowfall than any part of the USA east of the Mississippi River, more snowfall than any non-mountainous region of the continental United States. Copper mining in Michigan Copper Island Keweenaw National Historical Park List of Copper Country mines List of Copper Country mills List of Copper Country smelters Burt, Williams A. and Hubbard, Bela Reports on the Mineral Region of Lake Superior, 113 pages. Carnahan, Arthur L; the Lake Superior Copper Country, National Magazine, December, 1905 Harrison, Jim. "Imprint: My Upper Peninsula". The New York Times. Retrieved November 30, 2013. Thurner, Arthur W. Strangers and Sojourners - A History of Michigan's Keweenaw Peninsula ISBN 0-8143-2396-0
Isle Royale is an island of the Great Lakes, located in the northwest of Lake Superior, part of the U. S. state of Michigan. The island and the 450 surrounding smaller waters make up Isle Royale National Park; the island is 45 miles long and 9 miles wide, with an area of 206.73 square miles, making it the largest natural island in Lake Superior, the second largest island in the Great Lakes, the third largest in the contiguous United States, the 33rd largest island in the United States. Isle Royale is defined by the United States Census Bureau as Census Tract 9603 of Keweenaw County, Michigan; as of the 2000 census there was no permanent population. After the island was made a national park, some existing residents were allowed to stay, a few leases are still in effect. Ferries from Michigan and Wisconsin land at Rock Harbor on the eastern end of the island. Ferries from Minnesota run to Windigo on the western end, which has a visitor center and campground. In 1875, Isle Royale was set off from Keweenaw County, as a separate county, "Isle Royale County".
In 1897, the county was dissolved, the island was reincorporated into Keweenaw County. The highest point on the island is about 800 feet above lake level. Isle Royale is within about 15 miles of the Canadian and Minnesotan shores of the lake, is 56 miles from the Michigan shore, on the Keweenaw Peninsula. There are seasonal passenger ferry services to the island from Minnesota. There is a seasonal sea plane service. There are no roads on the island, wheeled vehicles or devices, other than wheelchairs, are not permitted. Rock Harbor has wheeled carts available to move personal belongings from the Rock Harbor marina to the cabins and hotel; the National Park Service employs tractors and a few World War II jeeps to move items around the ranger station area at Windigo, Rock Harbor, Mott Island. Topsoil tends to be thin, which favors trees that have horizontal root patterns such as balsam fir, white spruce, black spruce. Siskiwit Lake is the largest lake on the island, it has cold, clear water, low in nutrients.
Siskiwit Lake contains several islands, including Ryan Island, the largest, which contains Moose Flats, a seasonal pond, which contains Moose Boulder. When Moose Flats is a pond, Moose Boulder becomes the largest island in the largest lake on the largest island in the largest lake on the largest island in the largest lake in the world. Chicken Bone Lake Lake Desor Feldtmann Lake Intermediate Lake Lake Ritchie Sargent Lake There is no reliable climate data for Isle Royale. Isle Royale boasts the purest copper to be found anywhere in the world. A freak volcanic event in the distant past twisted the copper-bearing bedrock above the water line, which permitted all sulfur impurities to burn away in the open air; the island is composed of ridges, running southwest-to-northeast. The main ridge, Greenstone Ridge, is over 1,000 feet in many places. Greenstone belts are exposed, with rounded stones of chlorastrolite known as greenstone, near and in the lake. According to the National Park Service, the north sides of the ridges tend to be steeper than the south sides.
Coastal areas were once submerged beneath prehistoric lake waters, contain many tumbled boulders and other large rocks. The island was a common hunting ground for native people from nearby Ontario. A canoe voyage of thirteen miles is necessary to reach the island's west end from the mainland. In prehistoric times, large quantities of copper were mined on Isle Royale and the nearby Keweenaw Peninsula; the region is scarred by ancient mine trenches up to 20 feet deep. Carbon-14 testing of wood remains found in sockets of copper artifacts indicates that they are at least 5700 years old. In Prehistoric Copper Mining in the Lake Superior Region, published in 1961, Drier and Du Temple estimated that over 750,000 tons of copper had been mined from the region. However, David Johnson and Susan Martin contend that their estimate was based on exaggerated and inaccurate assumptions. In 1670, a Jesuit missionary named Dablon published an account of "an island called Menong, celebrated for its copper." Menong, or Minong, was the native term for the island, is the basis for the name of the Minong Ridge on the island.
Isle Royale was given to the United States by the 1783 treaty with Great Britain, but the British remained in control until after the War of 1812, the Ojibwa peoples considered the island to be their territory. The Ojibwas ceded the island to the U. S. in the 1842 Treaty of La Pointe, with the Grand Portage Band unaware that neither they nor Isle Royale were in British territory. With the clarification to the Ojibwas of the 1842 Webster–Ashburton Treaty, signed before the Treaty of La Pointe, the Ojibwas re-affirmed the 1842 Treaty of La Pointe in the 1844 Isle Royale Agreement, with the Grand Portage Band signing the agreement as an addendum to the 1842 treaty. In the mid-1840s, a report by Douglass Houghton, Michigan's first state geologist, set off a copper boom in the state, the first modern copper mines were opened on the island. Evidence of the earlier mining efforts was everywhere, in the form of many stone hammers, some copper artifacts, places where copper had been worked out of the rock but left in place.
The ancient pits and trenches led to the discovery of many of the copper deposits that were mine
The Ontonagon Boulder is a 3,708 pound boulder of native copper found in the Upper Peninsula of Michigan, United States, now in the possession of the Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution. In 1843 the boulder was bought off of a local entrepreneur and shipped to Washington D. C; the boulder is a relic of Michigan's upper Peninsula and was well known to Native Americans in its location on the west branch of the Ontonagon River, in what is now Victoria Reservoir. According to the Keweenaw Bay Indian Community, the boulder was used by tribe members to make offerings to its manitou and to seek improvement in their health and well-being. Although many attribute the boulder to a relic of Michigan's copper boom, it was not a product of the boom but the reason for it; the Copper boom was only realized after the boulder had been moved to Washington D. C. While the exact origin of the Ontonagon Boulder is unknown, it has been determined that the boulder reached a location about 20 miles up river from Lake Superior, on the west branch of the Ontonagon River, via being dragged by a glacier.
In the early seventeenth century, Voyageurs traversing Lake Superior heard word of the massive solid copper boulder. Early stories of the boulder describe it as being over five tons and as large as a house. In 1667, the redoubtable Jesuit missionary Claude Dablon made his way up the Ontonagon and confirmed the existence of the fabled rock. In 1766, under the guidance of a party of Ojibwe, trader Alexander Henry the elder laid eyes on the rock, reported that he found it to be so pure and malleable that he was able to remove a large piece, estimated the boulder's weight at ten tons. In May of 1798, David Thompson recorded the following during his exploration of the "River Ontonoggan", "Learning from my men that a short distance up the river there was a mass of copper, we left our canoe and proceeded on foot to it. We tried to cut a chip from it, but it was too tough for our small axe." During a geological voyage around the perimeter of Michigan in 1820, Henry R. Schoolcraft first reached the mouth of the Ontonagon River on June 27.
Schoolcraft and his fellow voyagers, led by four Native Americans, journeyed up the Ontonagon River in two canoes. The next day they continued up the river. From there they traveled on foot until they reached the legendary boulder. Schoolcraft was disappointed with the boulder, finding it much smaller than legends claimed it to be. However, Schoolcraft reported that the rock was scarred by the chisels and axes of Native Americans He went on to describe it as 3'8" by 3'4" and estimated its weight at 2200 pounds. In 1826 the Treaty of Fond du Lac granted to the United States the rights to minerals exploration and mining within Ojibwe lands located north of the Prairie du Chien Line. In 1842 the Treaty of La Pointe ceded lands now parts of Upper Peninsula Michigan. After many failed attempts, the Ontonagon Boulder was removed in 1843, by Julius Eldred, a Detroit hardware-store merchant. Prior to extraction, Eldred purchased the rock from the local Chippewa for $150 in 1841, his first two expeditions were only able raise the boulder on skids.
In 1843 Eldred tried again. This time he discovered the boulder, that he had bought from the local Native Americans, now belonged to a group of miners from Wisconsin, who had located the land under a permit issued directly by the Secretary of War. With no other choice Eldred paid an additional $1,365 for ownership of the rock he had purchased. After paying for his prize twice and his crew of 21 men, using a capstan, lifted the boulder 50 feet to the top of the adjacent bluff, it took a week to get to the top of the bluff. They cut a swath though the woods and laid out a short stretch of rails, they would push the railcar to the end of the short line, pick up the rails from behind, place them in front of the car again. Eldred and his men did this for four miles before reaching the bottom of the rapids, where the boulder was loaded onto a raft. Once the raft reached the mouth of the Ontonagon River it was loaded onto a schooner, which sailed to Copper Harbor. Eldred's victory was short-lived, because when they arrived in Copper Harbor Eldred was informed that the U.
S. Secretary of the Treasury had instructed the Secretary of War to claim federal ownership of the Ontonagon Boulder, ship it to Washington, D. C. However, Eldred was able to delay giving the boulder to the federal government, in the dead of night he hoisted it onto the deck of a waiting schooner, he sailed to Sault portage, where the boulder was loaded onto another schooner, which took the boulder the rest of the way to Detroit. In Detroit, Eldred placed the legendary Ontonagon Boulder on public display, charging a cash admission. In 1847, Eldred and the federal government went to court fighting over ownership of the boulder. In the end, the government took the boulder, but paid Eldred $5,644.93 for "his time and expense in purchasing and removing the mass of native copper." The boulder remained in the possession of the War Department until 1860, when it was placed on public display in the Smithsonian Institution. In 1991 an assessment was initiated after the Keweenaw Bay Indian Community requested the return of the Ontonagon Boulder as a sacred object.
A preliminary analysis indicated that the tribe presented insufficient evidence to establish that the boulder fit the definition of
An ore is an occurrence of rock or sediment that contains sufficient minerals with economically important elements metals, that can be economically extracted from the deposit. The ores are extracted from the earth through mining; the ore grade, or concentration of an ore mineral or metal, as well as its form of occurrence, will directly affect the costs associated with mining the ore. The cost of extraction must thus be weighed against the metal value contained in the rock to determine what ore can be processed and what ore is of too low a grade to be worth mining. Metal ores are oxides, silicates, or native metals that are not concentrated in the Earth's crust, or noble metals such as gold; the ores must be processed to extract the elements of interest from the waste rock and from the ore minerals. Ore bodies are formed by a variety of geological processes; the process of ore formation is called ore genesis. An ore deposit is an accumulation of ore; this is distinct from a mineral resource. An ore deposit is one occurrence of a particular ore type.
Most ore deposits are named according to their location, or after a discoverer, or after some whimsy, a historical figure, a prominent person, something from mythology or the code name of the resource company which found it. Ore deposits are classified according to various criteria developed via the study of economic geology, or ore genesis; the classifications below are typical. Mesothermal lode gold deposits, typified by the Golden Mile, Kalgoorlie Archaean conglomerate hosted gold-uranium deposits, typified by Elliot Lake, Ontario and Witwatersrand, South Africa Carlin–type gold deposits, including. Volcanic hosted massive sulfide Cu-Pb-Zn including. Stratiform arkose-hosted and shale-hosted copper, typified by the Zambian copperbelt. Stratiform tungsten, typified by the Erzgebirge deposits, Czechoslovakia Exhalative spilite-chert hosted gold deposits Mississippi valley type zinc-lead deposits Hematite iron ore deposits of altered banded iron formation Sudbury Basin nickel and copper, Canada The basic extraction of ore deposits follows these steps: Prospecting or exploration to find and define the extent and value of ore where it is located Conduct resource estimation to mathematically estimate the size and grade of the deposit Conduct a pre-feasibility study to determine the theoretical economics of the ore deposit.
This identifies, early on, whether further investment in estimation and engineering studies is warranted and identifies key risks and areas for further work. Conduct a feasibility study to evaluate the financial viability and financial risks and robustness of the project and make a decision as whether to develop or walk away from a proposed mine project; this includes mine planning to evaluate the economically recoverable portion of the deposit, the metallurgy and ore recoverability and payability of the ore concentrates, engineering and infrastructure costs and equity requirements and a cradle to grave analysis of the possible mine, from the initial excavation all the way through to reclamation. Development to create access to an ore body and building of mine plant and equipment The operation of the mine in an active sense Reclamation to make land where a mine had been suitable for future use Ores are traded internationally and comprise a sizeable portion of international trade in raw materials both in value and volume.
This is because the worldwide distribution of ores is unequal and dislocated from locations of peak demand and from smelting infrastructure. Most base metals are traded internationally on the London Metal Exchange, with
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard chalk and wallboard. A massive fine-grained white or tinted variety of gypsum, called alabaster, has been used for sculpture by many cultures including Ancient Egypt, Ancient Rome, the Byzantine Empire and the Nottingham alabasters of Medieval England. Gypsum crystallizes as beautiful translucent crystals of selenite, it forms as an evaporite mineral and as a hydration product of anhydrite. The Mohs scale of mineral hardness defines hardness value 2 as gypsum based on scratch hardness comparison; the word gypsum is derived from the Greek word γύψος, "plaster". Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum used for various purposes, this dehydrated gypsum became known as plaster of Paris. Upon addition of water, after a few tens of minutes plaster of Paris becomes regular gypsum again, causing the material to harden or "set" in ways that are useful for casting and construction.
Gypsum was known in Old English as spærstān, "spear stone", referring to its crystalline projections.. In the mid-18th century, the German clergyman and agriculturalist Johann Friderich Mayer investigated and publicized gypsum's use as a fertilizer. Gypsum may act as a source of sulfur for plant growth, in the early 19th century, it was regarded as an miraculous fertilizer. American farmers were so anxious to acquire it that a lively smuggling trade with Nova Scotia evolved, resulting in the so-called "Plaster War" of 1820. In the 19th century, it was known as lime sulfate or sulfate of lime. Gypsum is moderately water-soluble and, in contrast to most other salts, it exhibits retrograde solubility, becoming less soluble at higher temperatures; when gypsum is heated in air it loses water and converts first to calcium sulfate hemihydrate, and, if heated further, to anhydrous calcium sulfate. As for anhydrite, its solubility in saline solutions and in brines is strongly dependent on NaCl concentration.
Gypsum crystals are found to contain hydrogen bonding. Gypsum occurs in nature as flattened and twinned crystals, transparent, cleavable masses called selenite. Selenite contains no significant selenium. Selenite may occur in a silky, fibrous form, in which case it is called "satin spar", it may be granular or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A fine-grained white or tinted variety of gypsum, called alabaster, is prized for ornamental work of various sorts. In arid areas, gypsum can occur in a flower-like form opaque, with embedded sand grains called desert rose, it forms some of the largest crystals found in nature, up to 12 m long, in the form of selenite. Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks. Deposits are known to occur in strata from as far back as the Archaean eon. Gypsum is deposited from lake and sea water, as well as in hot springs, from volcanic vapors, sulfate solutions in veins.
Hydrothermal anhydrite in veins is hydrated to gypsum by groundwater in near-surface exposures. It is associated with the minerals halite and sulfur. Gypsum is the most common sulfate mineral. Pure gypsum is white, but other substances found as impurities may give a wide range of colors to local deposits; because gypsum dissolves over time in water, gypsum is found in the form of sand. However, the unique conditions of the White Sands National Monument in the US state of New Mexico have created a 710 km2 expanse of white gypsum sand, enough to supply the construction industry with drywall for 1,000 years. Commercial exploitation of the area opposed by area residents, was permanently prevented in 1933 when president Herbert Hoover declared the gypsum dunes a protected national monument. Gypsum is formed as a by-product of sulfide oxidation, amongst others by pyrite oxidation, when the sulfuric acid generated reacts with calcium carbonate, its presence indicates oxidizing conditions. Under reducing conditions, the sulfates it contains can be reduced back to sulfide by sulfate-reducing bacteria.
Electric power stations burning coal with flue gas desulfurization produce large quantities of gypsum as a byproduct from the scrubbers. Orbital pictures from the Mars Reconnaissance Orbiter have indicated the existence of gypsum dunes in the northern polar region of Mars, which were confirmed at ground level by the Mars Exploration Rover Opportunity. Commercial quantities of gypsum are found in the cities of Grajaú in Brazil. Large open pit quarries are located in many places including Fort Dodge, which sits on one of the largest deposits of gypsum in the world, Plaster City, United States, East Kutai, Indonesia. Several small mines exist in places such as Kalannie in Western Australia, where gypsum is sold to private buyers for additions of calcium and sulfur as well as reduction of aluminum toxicities on soil for agricultural purposes. Crystals of gypsum u
Lustre or luster is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux, meaning "light", implies radiance, gloss, or brilliance. A range of terms are used to describe lustre, such as earthy, metallic and silky; the term vitreous refers to a glassy lustre. A list of these terms is given below. Lustre varies over a wide continuum, so there are no rigid boundaries between the different types of lustre; the terms are combined to describe intermediate types of lustre. Some minerals exhibit unusual optical phenomena, such as asterism or chatoyancy. A list of such phenomena is given below. Adamantine minerals possess a superlative lustre, most notably seen in diamond; such minerals are transparent or translucent, have a high refractive index. Minerals with a true adamantine lustre are uncommon, with examples being cerussite and cubic zirconia. Minerals with a lesser degree of lustre are referred to as subadamantine, with some examples being garnet and corundum.
Dull minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating a Lambertian reflector. An example is kaolinite. A distinction is sometimes drawn between dull minerals and earthy minerals, with the latter being coarser, having less lustre. Greasy minerals resemble grease. A greasy lustre occurs in minerals containing a great abundance of microscopic inclusions, with examples including opal and cordierite, jadeite. Many minerals with a greasy lustre feel greasy to the touch. Metallic minerals have the lustre of polished metal, with ideal surfaces will work as a reflective surface. Examples include galena and magnetite. Pearly minerals consist of thin transparent co-planar sheets. Light reflecting from these layers give them a lustre reminiscent of pearls; such minerals possess perfect cleavage, with examples including stilbite. Resinous minerals have the appearance of chewing gum or plastic. A principal example is amber, a form of fossilized resin.
Silky minerals have a parallel arrangement of fine fibres, giving them a lustre reminiscent of silk. Examples include asbestos and the satin spar variety of gypsum. A fibrous lustre has a coarser texture. Submetallic minerals are duller and less reflective. A submetallic lustre occurs in near-opaque minerals with high refractive indices, such as sphalerite and cuprite. Vitreous minerals have the lustre of glass; this type of lustre is one of the most seen, occurs in transparent or translucent minerals with low refractive indices. Common examples include calcite, topaz, beryl and fluorite, among others. Waxy minerals have a lustre resembling wax. Examples include chalcedony. Asterism is the display of a star-shaped luminous area, it is seen in some rubies, where it is caused by impurities of rutile. It can occur in garnet and spinel. Aventurescence is a reflectance effect like that of glitter, it arises from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they influence the material's body colour.
In aventurine quartz, chrome-bearing fuchsite makes for a green stone and various iron oxides make for a red stone. Chatoyant minerals display luminous bands; such minerals are composed of parallel fibers, which reflect light into a direction perpendicular to their orientation, thus forming narrow bands of light. The most famous examples are tiger's eye and cymophane, but the effect may occur in other minerals such as aquamarine and tourmaline. Color change is most found in alexandrite, a variety of chrysoberyl gemstones. Other gems occur in color-change varieties, including sapphire, spinel. Alexandrite displays a color change dependent upon light, along with strong pleochroism; the gem results from small-scale replacement of aluminium by chromium oxide, responsible for alexandrite's characteristic green to red color change. Alexandrite from the Ural Mountains in Russia is green by red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light.
The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is rare. Iridescence is the'play' or'fire' of rainbow-coloured light caused by thin regular structures or layers beneath the surface of a gemstone. Similar to a thin film of oil on water, these layers interfere with the rays of reflected light, reinforcing some colours and cancelling others. Iridescence is seen at its best in precious opal. Schiller, from German for "color play", is the metallic iridescence originating from below the surface of a stone that occurs when light is reflected between layers of minerals, it is seen in moonstone and labradorite and is similar to adularescence and aventurescence